In such a structure, the expansion chamber and the water box are generally interconnected by a degassing conduit which runs from the top of the water box and opens out in the expansion chamber. It is advantageous for said conduit to open out below the normal level of liquid in the expansion chamber, since this helps to reduce the risks of air or gas being drawn into the conduit when the motor is stopped.
It is also advantageous for the degassing conduit to open out tangentially into the expansion chamber (which is generally of substantially circular section) or at least at a small angle to the wall of the chamber, since this leads to the formation of an eddy or a vortex in the chamber, thereby increasing the period of time for which the bubbles pass through the liquid which favours separation of gas and liquid, and hence provides improved degassing of the liquid.
However, it has been observed in practice that the degassing efficiency of such a water box and expansion chamber structure is lower than expected, in the main because the bubbles of air or gas are drawn back into the cooling circuit via the suction orifice which is generally at the bottom of the expansion chamber and which leads either to the outlet tube from the heat exchanger or else to the water box.
Preferred embodiments of the present invention avoid this drawback in a simple, cheap and efficient manner.